The present invention relates to the mounting of circuit card assemblies (CCA's) in a chassis or enclosure and to the maintenance of proper operating temperatures of mounted CCA's. More specifically, the present invention relates to heat transfer structures for removing operational heat from a circuit card through conduction of the heat to the surrounding chassis during operation.
It is common for CCA's to be mounted in a structure that is referred to as a chassis 13, FIG. 1. The chassis includes a number of parallel guide channels 14 for accommodating a number of CCA's 10. The individual cards 10 are slid into the chassis 13 along the channels 14 and electrically connected to the unit at the backplane. It is important in many applications to insure that each of the cards 10 are held in place in the channels 14 to ensure proper electrical connection, to improve heat conduction and to prevent damage to the cards which can arise from motion. Movement of the card can cause connections to the card to be broken resulting in card and/or system failure. A common method of securing circuit cards in place is to use a retaining device 15 which expands to create a tight fit within the channel 14, such as a wedge lock which is expanded once the card 10 is positioned in a channel 14 to wedge the card in place against the walls of the channel.
The use of multiple circuit cards and high-power or densely packed components can often result in the production of heat which cannot be easily dissipated. Excess heat will create temperatures above a normal or acceptable operating temperature range with the potential for improper operation, card failure or system failure. The individual components on a circuit card produce heat that must be conveyed away from the components and from the circuit card so that the temperatures of the components does not exceed their maximum operating temperature. Because of the amount of heat generated by the components, and the closely spaced circuit cards and limitations on the chassis, which may include sealing the chassis due to harsh environmental conditions, such as military and/or aerospace applications, the heat must be removed by conduction. As the sizes of the components become smaller and the spacing becomes tighter, the power dissipation per area increases and the heat removal problem becomes greater.
Heat can be conducted away from the individual components along the plane of the printed circuit card. As illustrated in FIG. 2, a printed circuit card 10 can include metal strips 11 along one or more edges of the card which make contact with a heat transfer frame 12 attached to the card to conduct heat from the circuit card to the chassis 13. CCA's designed for use in high-temperature environments often also incorporate heat dissipation channels in the circuit card to conduct heat away from individual components along the plane of the card to the heat dissipation strips 11 along the edge of the circuit card. These heat conducting strips 11, often metal pads, are located on an outer surface of the card 10 and are in contact with a heat dissipation frame 12, FIG. 3, which contacts one side of the parallel channel 14 of the chassis 13 when the card is mounted in the chassis channel 14 and held in place with a wedge lock device 15. As illustrated in FIG. 4, the heat dissipation frame is located on the side opposite from the wedge lock device 15 so that the frame is in direct contact with the wall of the channel 14 in the chassis 13.
The heat dissipation frame 12 may be an open frame which contacts the edge of the card or can include planer elements 18 which may be shaped to come in contact with one or more components 19 on the card 13 to directly conduct heat away from the components 19 to the frame 12. A portion of the frame is positioned in contact with the chassis 13 so as to conduct heat from the frame 12 to the chassis. The contact between the frame and the chassis is maintained by the wedge lock device 15. However, the frame is only in contact with one side of the channel, thereby limiting the conductive channel for the removal of heat.
As illustrated in FIGS. 4A through 4D, although the wedge lock 15 is in contact with the heat dissipation frame 12 and the chassis 13, it is a poor conductor of heat because the bottom segments 16 and top segments 17 of the wedge lock are not in good thermal contact with each other. FIGS. 4A and 4B illustrate the wedge lock before expansion. Bottom segments 16 are connected to the card and but can slide laterally when the wedge lock is expanded as illustrated in FIGS. 4C and 4D. Bottom segment 16a is attached to the card 10 and does not move laterally. When the wedge lock 15 is expanded to secure the card 10 in position, the top segments 17 are pushed up away from the bottom segments 16 and also move laterally. The resulting spacing between segments and the spacing between upper segments 17 and the card 10, results in a loss of thermal conductivity. The bottom 16 of the wedge lock contacts the heat frame 12 but the top of the wedge lock does not conduct heat well to the opposite side of the channel of the chassis 13. As illustrated in FIG. 4B, the wedge lock 15 has a hollow interior 9 to accommodate the central spindle 8 of the wedge lock. The hollow interior 9 of the wedge lock 15 also reduces thermal conductivity.